feat(search-plugin): add namespace option

[cheng-kang]

Issue

Local storage stores data per domain. [1]

It's unique per protocol://host:port combination.

Take the example in #697:

• https://ramblerw.github.io/blog-money
• https://ramblerw.github.io/python3

They are under the same domain https://ramblerw.github.io. Thus, they share the same search index data.

This PR

Introduces a new option: namespace, to avoid local storage collision.

If namespace is set, search index data will be saved under a namespaced key, e.g. docsify.search.index/docsify-test-namespace.

Fixes #697

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[libotony]

I am having the same problem, when will this feature be released?